1. Technical Field
The present invention relates to toner, a development agent, and an image forming apparatus.
2. Background Art
In electrophotography, electrostatic images (latent images) are formed on an image bearing member (typically a photoreceptor) and developed with toner to form visible toner images. The thus-formed toner image is transferred onto a transfer medium, typically paper, and thereafter fixed thereon by heating, etc.
Such toner contains a binder resin formed of, for example, a thermal plastic resin derived from petroleum such as a styrene-acrylic resin or a polyester resin. However, due to environmental concerns of late, using a biodegradable resin derived from biomass made from a recyclable resource starts to gain attention to reduce the burden on the environment at the time of disposal.
JP-H04-179967-A discloses using a microbial aliphatic polyester as such a biodegradable resin. However, if the microbial aliphatic polyester is used as a resin for toner, the softening temperature thereof becomes high because of its high crystallinity. As a consequence, the fixing temperature of the toner is inevitably high, which is disadvantageous in terms of energy-saving.
JP-2597452-B1 (JP-H06-289644-A) discloses a method of lowering the softening point of toner containing a biodegradable resin by adding a plant wax in a large amount to lower the fixing temperature of the toner. However, the toner easily agglomerates due to the wax component contained therein. Accordingly, the productivity suffers and the fluidity of the toner deteriorates, which has a negative impact on the toner transferability in a development device.
JP-2006-91278-A and JP-2006-285150-A disclose using a binder resin containing two kinds of resins having different softening points and a biodegradable resin (polylactic acid). The resin having a lower softening point serves to link the resin having a higher softening point and the biodegradable resin, so that the biodegradable resin is uniformly dispersed in the binder resin to obtain toner having a good low temperature fixability and fixing stability.
However, if the blending ratio of the biodegradable resin is designed to be high, the dispersion of the biodegradable resin tends to deteriorate. This leads to degradation of the developability due to variation of the charging power, which has an adverse impact on the durability. For this reason, the blending ratio of the biodegradable resin is unavoidingly extremely low, i.e., around 20% by weight, which is not sufficient to lessen the burden on the environment.
In addition, such toner is prepared by melt-kneading a binder resin and a blending agent such as a colorant, a charge control agent, and an offset resistance agent followed by pulverization and classification of the thus-obtained toner composition. However, the toner composition is required to be pulverizable and classifiable by an economically-affordable device. Moreover, the melt-kneaded toner composition must be sufficiently brittle. Accordingly, the selection of the toner material is limited, which inhibits furthermore improvement of the low temperature fixability.
Furthermore, the toner composition is pulverized to form toner particles, resulting in production of toner having a wide particle size distribution although toner having a sharp particle size distribution is suitable to obtain photocopy images having good definition and gradation. For this reason, fine particles having a particle diameter of 5 μm or less and coarse particles having a particle diameter of 20 μm or more are removed by classification, which invites an extremely low yield.
In addition, the pulverization method is disadvantageous in terms of uniform dispersion of a blending agent such as a colorant and a charge control agent in a thermoplastic resin. Unless such a blending agent is uniformly dispersed, the fluidity, the developability, the durability, and the image quality are adversely affected.
JP-3344214-B1 (JP-H09-319144-A) and JP-3455523-B1 (JP-2002-284881-A) disclose methods of granulating toner particles by dispersing a resin solution in which a binder resin is preliminarily dissolved in an organic solvent in an aqueous medium although no biodegradable resin is used. Such methods obviate the need for classification to obtain uniform particles.
However, if polylactic acid, which is made from plant resources, widely used, and available without difficulty is polymerized by using a single monomer, the crystallinity is so high that the solubility of the polylactic acid is extremely low. Therefore, using the method mentioned above including granulation in an aqueous medium after dissolution in an organic solvent is not suitable for such a polylactic acid.
A polylactic acid can be dissolved in an organic solvent more easily by using a monomer mixture of L-form and D-form, which are optical isomers of the polylactic acid instead of a simple monomer of L-form or D-form while changing the ratio of L-form/D-form to lower the crystallinity.
However, considering that it is difficult to control the molecular weight of polylactic acid and the molecular chain via ester linkage is only carbon atom (N=1), toner having satisfactory properties is not easily obtained by polylactic acid only.
One thinkable way to solve this issue is using a mixture of a polylactic acid and a second resin other than a polylactic acid to secure properties required for toner. However, even if the issues about the crystallinity and the solubility in an organic solvent are clear, it is extremely difficult to manufacture toner having satisfactory properties by using polylactic acid in combination with other resins because the compatibility and dispersability thereof with widely-used products in toner such as polyester resins and styrene and acrylic copolymers are extremely poor.
Among efforts to solve this issue, JP-2008-262179-A discloses using a block polymer having a polylactic acid unit and a polyester unit having no polylactic acid to improve the compatibility, thereby unifying the resin composition in toner, which leads to stable image output.
Moreover, JP-2010-14757-A discloses a method of forming a film on resin particles prepared by reaction of a polylactic acid and a material obtained by reacting the polylactic acid with an elongating agent with other resin particulates to obtain polylactic acid toner having excellent thermal properties, high temperature stability, and transparency.
Although this method is successful about the high temperature stability and the transparency, toner using polylactic acid generally has a high melt-viscosity, which makes it difficult to manufacture toner having a low fixing temperature. This is not preferable in terms of energy-saving.
It is possible to lower the melt viscosity of polylactic acid by decreasing the molecular weight thereof. However, by simply decreasing the molecular weight of polylactic acid, residual monomers and oligomers increase, thereby reducing hydrolysis resistance of polylactic acid, which inhibits demonstration of sufficient storage stability of toner.
JP-2011-149999-A discloses a method of lowering the fixing temperature by adding an aliphatic acid amide as a fixing helping agent.
In this method, in granulation of toner particles by dispersing an oil phase in which toner materials are dissolved or dispersed in an organic solvent in an aqueous medium, the aliphatic acid amide is melted in the organic solvent or an aqueous medium, which makes it difficult to obtain toner having a small particle diameter with a sharp particle size distribution.
In addition, widely-used polylactic acid is known to be not or little compatible with other organic materials so that when polylactic acid is used in combination with a fixing helping agent, the melt-viscosity of the toner is not sufficiently lowered by using the fixing helping agent. Also, since it is not easy to control arrangement of the fixing helping agent in toner particles and resultantly the fixing helping agent is exposed to the surfaces of the toner particles, agglomeration tends to occur in a development device due to mechanical stress.
As described above, it is not easy to use a plant resin as the main component of the binder resin of toner and the blend ratio of a plant resin is limited in the method in which part of the binder resin is replaced with the plant resin. Therefore, a technology to blend a plant resin in a higher ratio without a negative impact on the properties of the binder resin of toner is demanded.